1. Field of the Invention
This invention relates to an electromagnetic sensing device. More particularly, this invention relates to an electromagnetic sensing device suitable for avoiding subterranean obstacles such as utility pipelines during underground operations such as drilling. Although not so limited, the system is particularly suitable for use in connection with Horizontal Directional Drilling (HDD) machinery. The system allows for the detection of underground utilities at a distance sufficient to allow the drill to avoid these obstacles. Although particularly suitable for use in directional drilling operations, it may be used in any application where space is limited and sensing of objects in opaque materials is required. The electromagnetic sensor design is a novel implementation of a stepped frequency continuous ground penetrating radar system designed to fit inside the drill head of a horizontal directional drill or in other applications where space requirements are restrictive. The system comprises the electronics to generate and receive the radar signals, an adaptive antenna designed specifically for various soil conditions, communications and power electronics to allow the radar to be controlled via a single conductor in the drill-string and a human machine interface (HMI) that performs display, storage and processing functions.
In operation, the radar generates continuous wave frequencies over a 400 MHz to 1000 MHz bandwidth. This signal is applied to the terminals of an electronically matched antenna that radiates energy ahead of the drill string. The scattered energy from the object for each frequency is received by the radar sensor and converted to a digital signal. This data is calibrated and converted to a spatial domain and then transmitted to the HMI by means of the drill string. The HMI provides a simple A-scope radar interface that tracks the targets ahead of the drill. The HMI also interfaces with the drill hardware to allow for automatic shutdown of the drill if utilities or other objects are encountered in the drill path.
2. Description of Related Art
Guided directional drilling equipment is being used more and more frequently for the installation of underground utilities. These trenchless installations offer significant advantages over trenching operations, including ease of installation in inaccessible areas and lower costs. However, with this installation ease and lower cost, there is the potential hazard of cutting existing underground utilities and the significant cost incurred for repair and loss of service. Even with the use of surface locating technology and one call services, existing utilities are regularly cut. In 1993 there were more than 104,000 hits or third party damage to gas pipelines in the United States with a total cost exceeding $86 million. Even though not all these utility hits were the result of directional drilling operations, the magnitude of the problem is evident. Companies responsible for cuts are also being charged for revenue loss in addition to repair costs. Thus, to reduce the risk of utility damage, it is essential to develop new techniques, other than standard surface locating methods, to locate utilities in the path of and adjacent to new guided drill bores.
Other than standard pipe and cable locators, the most commonly applied geophysical technique for locating underground utilities is ground penetrating radar (GPR). Surveys are normally conducted from the surface and the location and depth of potential utilities are determined from an analysis of reflected energy. Other techniques that have been used include magnetic field sensors, seismic or acoustic techniques, and electromagnetic induction sensors. All these techniques are most commonly applied from the surface and, as such, provide no information on conditions in the immediate vicinity of the drill as drilling progresses. Errors in lateral and depth locations result in utility cuts, both as the drill advances and when the hole is subsequently reamed. According to users, most utility hits occur on the back ream. While utilities are missed as the pilot bore is drilled, they are close enough laterally that they are cut as the reamer is pulled back through the hole. Hence, any drill head technique needs to be able to look both ahead of the drill and to the side. This capability will enable avoidance of utilities directly in its path, both when the hole is initially drilled and when the hole is reamed during the product installation phase.
While not applicable in all soil types, GPR provides one of the fastest and most accurate determinations of object location of any geophysical sensing technique. This rapid data acquisition feature of GPR is essential, because with an advancing drill stem, obstacle avoidance information must be acquired and evaluated rapidly. Furthermore, a means of recording the location of utilities during drilling must be provided. In addition to providing immediate assistance, these data provide input for a database of as-built conditions of pre-existing utilities.
As important as high signal to noise ratio data collection is the presentation of data in a manner that is easily interpreted. Simultaneous rotation and advance of the drill string typically result in data that do not necessarily appear the same as that normally collected from surface surveys. Analyses of the expected returns of different utilities at different relative orientations is thus desirable. Because of the rapid advance rate, the drill operator must be able to identify and react to utilities quickly. Alternatively, the drill may be linked to interlock circuits that provide an automatic shutdown if utilities are approached too rapidly for operator intervention, thereby requiring processing and information display in real time.